1. Image forming apparatuses are constituted to use a toner to develop an electrostatic latent image formed on a latent-image carrier and transfer the developed image to a transfer member, such as paper, so that a transferred image of the exposed electrostatic latent image positioned on the latent-image carrier is obtained on the transfer member.
FIG. 10 is a schematic view showing an intermediate transfer type full-color image forming apparatus as an example of the conventional image forming apparatus.
As shown in FIG. 10, the image forming apparatus 101 is constituted such that an image is exposed on a latent-image carrier (hereinafter sometimes called an "OPC") 102 so that an electrostatic latent image is formed. Moreover, the electrostatic latent image on the OPC 102 is sequentially (the order of colors may be determined arbitrarily) developed by yellow, magenta, cyan and black development units 103, 104, 105 and 106 so as to be formed into a visible image. Then, color matching of the developed image on the OPC 102 is performed by a transferring unit 107, and then transferred to a transfer paper 108 which is one example of transfer members. Then, the transfer image is fixed by a fixing unit 109. As a result, a required image is obtained on the transfer paper 108.
After the developed image has been transferred to the transfer paper 108, residual toner T' left on the OPC 102 is removed by a cleaning blade 110 so as to be gathered into a residual toner box 111.
Toner T for use in the conventional image forming apparatus 101 is described below. Toner T supplied from the toner supply members 103c, 104c, 105c and 106c of the development units and placed on the toner carriers 103a, 104a, 105a and 106a is, by the toner-regulating members 103b, 104b, 105b and 106b, formed into a uniform layer and uniformly electrified. Then, toner T is moved to the OPC 102. To reliably move toner T to the OPC 102 through spaces between the toner carriers 103a, 104a, 105a and 106a and the toner blades 103b, 104b, 105b and 106b, toner T must have satisfactory fluidity and excellent electrification characteristic. Therefore, conventional toner T has a constitution that external additives 13 composed of silica (SiO.sub.2) are allowed to adhere to the surfaces of mother particles 12 made of a resin as shown in FIG. 11 to improve the fluidity and electrification characteristics thereof.
Toner T produced such that the external additives 13 are allowed to adhere to the mother particles 12 also includes the external additives 13 liberated from the mother particles 12. The liberated external additives 13 are relatively hard as compared with the mother particles 12. When the liberated external additives 13 contained in residual toner T' are removed by the cleaning blade 110, the liberated external additives 13 are fixed to a contact portion (a nip portion) 102a of the OPC 102 with the cleaning blade 110 as shown in FIG. 12 so that the external additives 13 remain. As a result, so-called filming occurs. If filming occurs, an image having a satisfactory quality cannot be obtained.
Therefore, a technique for preventing occurrence of filming has been suggested in Japanese Patent Publication No. 7-99438 by reducing coagulation (aggregation) of the external additives 13 liberated from the mother particles 12 of toner T for use in the image forming apparatus 1.
According to the foregoing disclosure, occurrence of the filming at the cleaning blade 110 due to the external additives 13 can be somewhat reduced.
The inventors of the present invention has variously studied about filming, thus resulting in a fact that external additives 13 allowed to adhere to the mother particles 12 of toner T also can fix to the cleaning blade 110 so that filming occurs. That is, stress is repeatedly exerted on the external additives 13 allowed to adhere to the mother particles 12 when a contact development is performed in which the external additives 13 are brought into contact with the OPC 102 and when a contact transfer is performed in which the external additives 13 are brought into contact with the transferring unit 107 during the process for developing an electrostatic latent image on the OPC 102 and the process for transferring a developed image on the OPC 102. As a result of the stress, the external additives 13 are liberated from the mother particles 12.
Even if coagulation (aggregation) of the external additives 13 liberated from the mother particles 12 in toner T is reduced as disclosed in the foregoing disclosure, the external additives 13 liberated from the mother particles 12 owning to the stress undesirably remain in the nip portion 102a of the OPC 102. Thus, external additives 13 are fixed to the nip portion 102a. Thus, filming occurs.
If the external additives 13 in the form of the coagulation do not exist in toner T, existence of the liberated external additives 13 having a small particle size in a large amount causes secondary coagulation of the external additives 13 to occur. The secondary coagulation is caused from electrostatic coagulation which occurs during contact development with the OPC 102 when the development process is performed and contact transfer with the transferring unit 107. Stress of toner T which is exerted during the contact development with the OPC 102 is usually larger than stress in the development unit. The development unit for use in a high image quality development system is generally arranged to rotate at different peripheral speed with respect to the OPC 102. Therefore, a fact has been detected that great stress is exerted on toner T and, thus, secondary coagulation of the external additives 13 easily occurs.
As described above, the disclosed toner cannot prevent occurrence of coagulation caused from the secondary coagulation of the external additives 13 liberated from the mother particles 12 in toner T. Therefore, occurrence of filming cannot satisfactorily and effectively be prevented.
In addition, there is a concern that the external additives are liberated from the mother particles when stress is repeatedly exerted on toner during contact development and contact transfer. Therefore, the lifetime of toner is limited. That is, elongation of the lifetime of toner cannot be expected.
2. FIG. 20 is a schematic view showing an intermediate transfer type full-color image forming apparatus as an example of the conventional image forming apparatus.
As shown in FIG. 20, the image forming apparatus 201 is constituted such that an image is exposed on to a latent-image carrier (hereinafter sometimes called an "OPC") 202 so that an electrostatic latent image is formed. Moreover, the electrostatic latent image on the OPC 202 is sequentially (the order of colors may be determined arbitrarily) developed by yellow, magenta, cyan and black development units 203, 204, 205 and 206 so as to be formed into a visible image. Then, color matching of the developed image on the OPC 202 is performed by a transferring unit 207, and then transferred to transfer paper 208 which is one of transfer members. Then, the transfer image is fixed by a fixing unit 209. As a result, a required image is obtained on the transfer paper 208.
After the developed image is transferred to the transfer paper 208, residual toner T' left on the OPC 202 is removed by a cleaning blade 210 so as to be gathered in a residual toner box 211.
Toner T for use in the conventional image forming apparatus 201 is described below. Toner T supplied from the toner supply members 203c, 204c, 205c and 206c of the development units and placed on the toner carriers 203a, 204a, 205a and 206a is, by toner-regulating members 203b, 204b, 205b and 206b, formed into a uniform layer and uniformly electrified. Then, toner T is moved to the OPC 202. To reliably move toner T to the OPC 202 through spaces between the toner carriers 203a, 204a, 205a and 206a and the toner blades 203b, 204b, 205b and 206b, toner T must have satisfactory fluidity and excellent electrification characteristic. Therefore, conventional toner T has a constitution that external additives 13 composed of silica (SiO.sub.2) are allowed to adhere to the surfaces of mother particles 12 made of a resin as shown in FIG. 11 to improve the fluidity and electrification characteristics thereof.
Toner T comprising the external additives 13 allowed to adhere to the mother particles 12 has the characteristics as shown in FIG. 21 that the mother particles 12 has higher adhesive property as compared with that of a heating member 209a of the fixing unit 209. On the other hand, the external additives 13 have low adhesive property as compared with that of the heating member 209a of the fixing unit 209. If toner T on the transfer paper 208 is toner T which smoothly adhere to the mother particles 12 of the external additives 13, the external additives 13 exist at an interface between the heating member 209a of the fixing unit 209 and toner T on the transfer paper 208 when the transferred image on the transfer paper 208 is fixed. Therefore, adhesion between the heating member 209a and toner T on the transfer paper 208 is considerably reduced. Therefore, so-called offset of toner T with which toner T on the transfer paper 208 adheres to the heating member 209a does not occur.
If toner T on the transfer paper 208 is toner T which does not smoothly adhere to the mother particles 12 of the external additives 13, substantially no external additives 13 exists at the interface between the heating member 209a and toner T on the transfer paper 208 during the fixing process. Therefore, the adhesion between the heating member 209a and toner T on the transfer paper 208 is undesirably enhanced. Therefore, offset of toner T occurs as shown in FIG. 21. Hence it follows that the transfer paper 208 undesirably wound to the heating member 209a.
Therefore, a technique has been suggested in Japanese Patent Publication No. 5-56501, in which toner is arranged such that specific inorganic fine particles having a separating function is mixed with toner T for use in the image forming apparatus 201. Thus, offset of toner occurring during the fixing process is prevented.
Toner disclosed as described above causes the specific inorganic fine particles exist between the surface of molten toner and the heating member during the fixing process. Thus, the separating characteristic of the specific inorganic fine particles prevents adhesion of toner T to the heating member. As a result, occurrence of offset can be prevented.
The inventors of the present invention has studied the offset of toner which occurs during the fixing process. As a result, toner T of the type having the external additives 13 allowed to adhere to the mother particles 12 is caused to be toner T forming the transferred image transferred to the surface of the transfer paper 208 and including toner in which the external additives 13 are liberated and inhibited from satisfactory adhesion to the mother particles 12. The reason for this lies in that stress is repeatedly exerted on the external additives 13 allowed to adhere to the mother particles 12 when a contact development is performed in which the external additives 13 are brought into contact with the OPC 202 and when a contact transfer is performed in which the external additives 13 are brought into contact with the transferring unit 207. As a result of the stress, the external additives 13 are liberated from the mother particles 12.
Even if the specific inorganic fine particles are mixed with toner as employed in the foregoing disclosure, the stress sometimes causes the specific inorganic fine particles to be liberated from the mother particles. Therefore, there is a concern that the offset of toner cannot effectively be prevented.
As described above, the disclosed toner cannot necessarily prevent liberation of the specific inorganic fine particles from the mother particles which occurs owning to the stress which is exerted during the contact development process and the contact transfer process. Therefore, occurrence of the offset of toner cannot satisfactorily and effectively be prevented. In addition, the disclosed toner cannot reliably prevent liberation of the specific inorganic fine particles from the mother particles. To prevent occurrence of the offset, the lifetime of toner is limited. That is, elongation of the lifetime of toner cannot be expected.
3. As shown in FIG. 26, a conventional development unit 301 is arranged to develop an electrostatic latent image on the surface of a latent-image carrier with toner. Toner in a toner-accommodating portion 302 moved to a toner supply member 304 by a toner-carrying member 303. Then, toner T is supplied to a toner carrier 305 by the toner supply member 304 so as to be held on the surface of the toner carrier 305. Moreover, toner T on the toner carrier 305 is formed into a uniform thin layer by a toner-regulating blade 306. Moreover, toner T is uniformly electrified, and then moved to a latent-image carrier 307. Toner T is used to develop an electrostatic latent image on the latent-image carrier 307 so as to be visualized.
Toner T for use in the conventional and usual development unit 301 is allowed to pass through a space between the toner carrier 305 and the toner-regulating blade 306 so as to be moved to the latent-image carrier 307 when toner T on the toner carrier 305 is formed into the uniform thin layer and uniformly electrified by the toner-regulating blade 306. Toner T must pass through the space between the toner carrier 305 and the toner-regulating blade 306 so as to be formed into the uniform thin layer and electrified uniformly so as to be moved to the latent-image carrier 307. Therefore, toner T must have satisfactory fluidity and excellent electrification characteristic. Therefore, as shown in FIG. 11, conventional toner T has a constitution that external additives 13 composed of silica (SiO.sub.2) are allowed to adhere to the surfaces of mother particles 12 composed of a resin. Thus, the required fluidity and electrification characteristic have been obtained.
The particle sizes of particles of toner T, however, vary considerably. Moreover, adhesion of the external additives 13 to the mother particles 12 is not always uniformly and sufficiently performed. Hence it follows that toner T having unsatisfactory fluidity and electrification characteristic is formed. If toner T of the foregoing type is moved to a contact portion (hereinafter also called a "nip portion") of the toner-regulating blade 306 with the toner carrier 305, toner T cannot pass through the space between the toner carrier 305 and the toner-regulating blade 306, as shown in FIG. 27. Thus, toner T is selectively left in the nip portion and retention of toner T occur. If toner T having unsatisfactory fluidity, electrification characteristic and large particle sizes is moved and retained in the nip portion. The retained toner T having the large particle size undesirably forms a movement stripe on the toner carrier 305.
If the foregoing movement stripe is formed on the toner carrier 305, only toner T having satisfactory fluidity, electrification characteristic and small and intermediate particle sizes is selectively moved to the latent-image carrier 307. Therefore, an excellent image quality cannot be realized.
4. As shown in FIG. 30, a conventional one-component development unit 401 uses a usual one-component developer such that a one-component developer composed of toner is used to develop an electrostatic latent image formed on the surface of a latent-image carrier. In the development unit 401, toner serving as the one-component developer in the toner-accommodating portion 402 is moved to a toner supply roller 404 by a toner-carrying member 403. Then, toner T is supplied to a developer carrier 405 by a toner supply roller 404 so as to be held on the surface of the developer carrier 405. Then, toner T on the developer carrier 405 is formed into a uniform thin layer and uniformly electrified by a toner-regulating blade 406 so as to be moved to the latent-image carrier 407. Toner T is used to develop an electrostatic latent image on the latent-image carrier 407 so as to be visualized.
Toner T for use in the conventional and usual development unit 401 is allowed to pass through a space between the developer carrier 405 and the toner-regulating blade 406 so as to be moved to the latent-image carrier 407 when toner T on the developer carrier 405 is formed into the uniform thin layer and uniformly electrified by the toner-regulating blade 406.
When the developer carrier 405 in a state in which the toner-regulating blade 406 has been made contact with the developer carrier 405 is rotated at high speed to obtain a large amount of image at high speed by the one-component development method, a portion of toner T cannot pass through the space between the developer carrier 405 and the toner-regulating blade 406, as shown in FIG. 31. The portion of toner T is sometimes and undesirably fixed to a contact portion (hereinafter called a "nip portion") 406a of the toner-regulating blade 406 in which the toner-regulating blade 406 is made contact with the developer carrier 405. The fixed toner T" causes unevenness to occur during the process of the developer carrier 405 to move toner. Therefore, the conventional one-component development method encounters frequent occurrence of unevenness of the density of a formed image in a form of a longitudinal stripe.
Toner T must pass through the space between the developer carrier 405 and the toner-regulating blade 406 so as to be formed into a uniform thin layer and uniformly electrified. Then, toner T is moved to the latent-image carrier 407. To achieve this, toner T must have satisfactory fluidity and excellent electrification characteristic. Therefore, as shown in FIG. 11, conventional toner T has a constitution that external additives 13 composed of silica (SiO.sub.2) are allowed to adhere to the surfaces of mother particles 12 composed of a resin. Thus, the required fluidity and electrification characteristic have been obtained.
The particle sizes of particles of toner T, however, vary considerably. Moreover, adhesion of the external additives 13 to the mother particles 12 is not always uniformly and sufficiently performed. Hence it follows that toner T having unsatisfactory fluidity and electrification characteristic is formed. If toner T of the foregoing type is moved to the nip portion 406a of the toner-regulating blade 406, also toner T of the foregoing type cannot pass through the space between the developer carrier 405 and the toner-regulating blade 406. As a result, toner T is undesirably fixed to the nip portion 406a of the toner-regulating blade 406. Thus, the fixed toner T" causes unevenness to occur in moving toner T as described above. As a result, unevenness in the density of the image in the form of a longitudinal stripe takes place.
Therefore, a development unit has been disclosed in Japanese Patent Laid-Open No. 6-11879, which is constituted such that a film-thickness-regulating member made contact with a rotative developer carrier which carries toner is worn owning to friction with the developer carrier before toner is fixed. Thus, it is attempted to prevent fixation of toner to the film-thickness-regulating member.
The disclosed development unit enables the portion of the film-thickness-regulating member, to which toner will be fixed, to be removed before the fixation of toner. As a result, fixation of toner hardly occurs.
The development unit disclosed as described above and constituted such that the film-thickness-regulating member is worn encounters limitation for use of the film-thickness-regulating member. As a result, the development unit cannot easily be used for a long time. That is, there arises a problem in that the durability of the development unit is unsatisfactory.
The development unit disclosed as described above requires the film-thickness-regulating member having a special shape and made of a special material. In addition, the film-thickness-regulating member is brought into contact with the developer carrier when the film-thickness-regulating member has been worn. Therefore, the film-thickness-regulating member must be pressed against the developer carrier by a pressing member. As a result, there arises a problem in that the film-thickness-regulating member has a too complicated structure and cost cannot easily be reduced.
Moreover, control is required such that pressing of the film-thickness-regulating member by means of the pressing member is performed substantially uniformly. It leads to a fact that the structures of the film-thickness-regulating member and the pressing member are excessively complicated. Thus, there arises a problem in that the foregoing members cannot easily be manufactured. Since the film-thickness-regulating member must have a special shape and a special material and the pressing member is required, the foregoing structure cannot easily be applied to the conventional development unit. Therefore, there arises a problem in that general versatility cannot be realized.
5. The reason why the above-described fixation of toner T' to the developer-regulating member 406 occurs is considered as follows. Toner T, in which the concentration of the external additives 13 is lower than a predetermined concentration, which has a low coverage of external additives and which has a small particle size is, by physical adhesive force, allowed to adhere to a contact portion between the developer-regulating member 406 and the developer carrier 405. Then, toner T is repeatedly slides and rubbed between the developer-regulating member 406 and the developer carrier 405 which moves at high speed. Thus, thermomechanical stress is exerted on toner T. Toner T having a low coverage of the external additives and a small particle size has a problem in that thermal deformation easily occurs because its thermal capacity is reduced according to the small volume. Moreover, movement caused from flow of toner does not easily occur because the coverage of the external additives is low. Therefore, when thermomechanical stress is, from outside, exerted on toner T having the low coverage of external additives is low and a small particle size, toner T cannot disperse the stress to the surrounding portions. Therefore, toner T is undesirably deformed. As a result, toner T having the deformed shape is joined to adjacent toner and the surface of the developer-regulating member 406. Thus, fixation of toner T occurs.
Fixed toner T" causes unevenness in the movement of toner similarly to the foregoing description. Thus, unevenness in the density of the images in the form of a longitudinal stripe takes place.
6. A conventional image forming apparatus of conventional type encounters a fact that the gradation expression is improved as the particle size of toner is reduced. Thus, the image quality can be improved, causing the resolution of a developed image on the OPC to be improved. On the other hand, a transferred image transferred from the OPC to the transfer member encounters a fact that the resolution realized by transfer excessively deteriorates as the particle size of toner is reduced.
A mono-color image forming apparatus, which is attempted to be capable of obtaining a transferred image having a high resolution even if the particle size of toner is reduced, has been suggested in Japanese Patent Laid-Open No. 3-170979. The image forming apparatus disclosed as described above is constituted to directly press transfer paper against a toner image developed on the OPC. Thus, the toner image is physically transferred on the transfer paper. As a result, flying and retention from transfer can be reduced to improve the efficiency in transferring. Thus, a transferred image having a high resolution can be obtained.
The reduction in the particle size of toner, however, causes its fluidity to deteriorate. As the fluidity of toner deteriorates, missing of an intermediate portion of a characteristic or a line occurs.
Therefore, the foregoing disclosure has disclosed a technique that toner is covered with silica (for example, mother particles of toner are added and covered with external additives (SiO.sub.2) as shown in FIG. 11 in spite of omission from the disclosure) to improve the fluidity of toner. Tests were performed to measure change in the fluidity and that in the ratio of missing of an intermediate portion in a line having a thickness of 300 .mu.m when the amount of silica, which is added to toner having a particle size of 7 .mu.m, has been changed from 0.2 wt % to 2.0 wt %. When the amount of silica, which is added, is made to be 0.4 wt % or larger, the ratio of missing of an intermediate portion can be lowered to be 5% or lower. As a result, missing of an intermediate portion cannot be recognized as a defective image by the unaided eyes. Thus, an image exhibiting excellent gradient and sharpness can be obtained.
Full color image forming apparatuses, such as color printers, have been developed in recent years. FIG. 41 is a schematic view showing an intermediate transfer type color printer which is an example of a conventional full color image forming apparatus.
Referring to FIG. 41, in the color printer 601, a print command signal (an image forming signal) supplied from a computer (not shown) is supplied to a control unit (not shown) of the color printer 601. As a result, rotations of the following units in predetermined directions are performed: an OPC 602, development units 603, 604, 605 and 606 for developing corresponding colors (yellow, magenta, cyan and black) (the order of the development units corresponding to the foregoing colors is arbitrarily), development rollers 603a, 604a, 605a and 606a which are toner carriers and an intermediate transfer medium (a drum may be substituted for an illustrated transfer belt) 607. Then, the outer surface of the OPC 602 is uniformly electrified to surface potential of V.sub.o by an electrifying roller 608 arranged to apply voltage V.sub.a.
Then, selective exposure to the outer surface of the OPC 602, which has uniformly been electrified, in accordance with image information of yellow, which is a first color, is performed by an exposing unit. Thus, an electrostatic latent image in yellow is formed. Then, only a development roller 603a of a development unit 603 for yellow is brought into contact with the OPC 602. Moreover, toner is electrified with development bias voltage V.sub.b of the development roller 603a so as to be moved to the OPC 602. Therefore, an electrostatic latent image for yellow on the OPC 602 is developed with toner so that a yellow toner image is formed on the OPC 602. The yellow toner image formed on the OPC 602 is primarily be transferred to the intermediate transfer medium 607 so that a yellow toner image is formed. At this time, a secondary transfer roller 607a and a cleaning blade 609 are brought to a state in which they are positioned apart from the intermediate transfer medium 607.
Residual toner T' is left on the OPC 602 after the primary transfer of the yellow toner image has been completed. Residual toner T' is removed by a cleaning blade 610 of the OPC 602 so as to be gathered in a residual toner box 611. Then, the OPC is destaticized by destaticizing light. Then, the exposing unit is again operated to perform selective exposure in accordance with image information of magenta which is a second color. Then, the development roller 603a of the development unit 603 is moved apart from the OPC 602. Moreover, only the development roller 604a of the development unit 604 is brought into contact with the OPC 602. As a result, an electrostatic latent image for magenta on the OPC 602 is developed so that a magenta toner image is formed on the OPC 602. Similarly to the process for forming the yellow image, the magenta toner image is primarily transferred to the intermediate transfer medium 607 so that a magenta toner image is formed. Then, residual toner on the OPC 602 is removed by the cleaning blade 610. Moreover, the OPC 602 is destaticized. Then, similar operations are performed for cyan, which is a third color and black which is a fourth color. Thus, the four colors are matched on the intermediate transfer medium 607 so that toner image in four colors is formed on the intermediate transfer medium 607.
After the toner image in the four colors has been formed on the intermediate transfer medium 607 owning to the primary transfer, the secondary transfer roller 607a is pressed against the intermediate transfer medium 607. Thus, the toner image in the four colors on the intermediate transfer medium 607 is transferred to the transfer member 612. Moreover, a cleaning blade 609 for the intermediate transfer medium 607 is brought into contact with the intermediate transfer medium 607. Then, residual toner T' left on the intermediate transfer medium 607 after the secondary transfer of the toner image to the transfer member 612 is remove by the cleaning blade 609. Similarly to the process for cleaning the OPC 602, residual toner T' is gathered in a residual toner box (not shown).
The toner image in the four colors formed on the transfer member 612 owning to the secondary transfer is allowed to pass through a fixing unit 613. Thus, the toner image is fixed to the surface of the transfer member 612. Then, paired discharge rollers (not shown) are rotated to discharge and accommodate the transfer member 612 having the fixed toner image in a case. Thus, a full color image is formed on the transfer member 612 by the color printer 601.
The transferring operation which is performed by the full color printer 601 encounters a problem in that unevenness in color occurs owning to missing of an intermediate portion when multilayered toner in the form of superimposition of four colors. In general, unevenness in color occurring owning to the missing of an intermediate portion can somewhat be obtained by enlarging the thickness of each color. However, the amount of exhaust toner is enlarged owning to residue from transfer. Thus, there arises a problem in that the cost cannot be reduced owning to wasteful use of toner.
Also the full color printer 601 may be constituted such that silica is added by 0.4 wt % or greater to overcome the problem of missing of an intermediate portion as disclosed in the foregoing disclosure. If rough paper is employed as the transfer member 612, simple increase in the amount of silica cannot overcome the problem of unevenness in color occurring owning to missing of an intermediate portion when transfer to the rough paper is performed.
Toner of a type coated with the external additive-synchronized toner in a sufficiently large amount encounters deterioration of toner after toner has been used for a long time. As a result, there arise problem in that transfer efficiency deteriorates and that stable color development cannot be performed.
As described above, the conventional image forming apparatus has been suffered from difficulty in efficiently transferring toner in the foregoing colors and stabilizing the color development in both of short time and long time aspects depending on the type of the transfer member 612 including rough paper.